QuestionHi Scott, I've got a '79 Spitfire that has the following problem when it gets hot (after about 1/2 hour of driving): I take my foot off the gas to switch gears or to stop, and the engine returns to idle VERY slowly, and sometimes it even revs up a little before returning to idle. Due to other reasons, I've had the Zenith carb rebuilt by my mechanic (who's a carb specialist), it's filled with oil, the throttle cable's been lubed, and my mechanic thinks the throttle return spring is OK. What would you think I should check or replace? (I had this problem before the carb rebuild as well).
AnswerA couple of questions to help me understand better; as it is likely 1 of 2 things: the throttle linkage or a pool of gas inside the intake manifold (carb is flooding)
1) when this occurs, have you ever opened the hood and looked at the position of the butterfly valve when this is happening? If the butterfly valve is still in the open position, and then slowly closing as the idle comes down, then something is "expanding" from the heat and causing the throttle return to bind when hot, which is either
a) kink in the cable
b) spring is losing tension when hot
c) the throttle shaft/butteryfly valve/carb body is out of alignment and binding.
2) When the engine is off, and you manually work the gas pedal, does the butterfly valve snap back to "closed" quickly?
3) If you manually work the butterfly valve directly on the carb itself, not using the throttle cable, does it snap back to closed quickly?
Do 2 and 3 after the engine is hot and having the problem; shut the car off and try 2 & 3.
If 3 is true, and 2 is false, then the problem is in the linkage. If 3 doesnt' quickly close, then the spring is shot or the shaft is binding when hot. If you can close the butterfly valve by hand easily, then add an extra spring or replace the spring.
Since you had the problem before rebuilding the carb, and if we assume your carb mechanic knows the basics on rebuilding ZS carbs, my gut is telling me its in the pedal/bar/cable. But there is another possibility
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79 is heavily emission controlled; I don't know how much you have stripped off or kept. You may have gas building up somewhere in the emissions circuit, and when hot, that gas is feeding the intake manifold when you close off the butterfly valve, and that is causing your issue. The emissions control system ties gas bowl vent, gas tank vents to the intake manifold, and if you have liquid gas pooling somewhere, you may have just enough to cause this problem; which could be caused by a mis-set carb float level, a plugged vent line, the possibilities are numerous.
If 1/2/3 aren't it, you may want to consider removing emissions connections to the intake manifold and run the car to see if the problem remains
Let me know what you find out with these tests; the more you can specifically describe the problem (i.e. actual RPMs, how many seconds, describe what you see moving, if anything on the engine when this occurs, is a lumpy running or smooth running when this happens, etc, the more it may trigger a thought on what t check next
